An accelerometer is one of the major sensors used in navigational systems, particularly inertial navigational systems, and on-board automotive safety control systems. Automotive examples of accelerometer use include various anti-lock braking systems, active suspension systems, and seat belt lock-up systems.
Generally, an accelerometer is a device which measures acceleration and, in particular, an accelerometer measures the force that is exerted when a moving body changes velocity. The moving body possesses inertia, which causes the body to resist the change in velocity. It is this resistance to a sudden change in velocity that is the origin of the force which is exerted by the moving body when it is accelerated. This force is proportional to the acceleration component in the direction of the movement, and therefore may be detected by an accelerometer.
In a typical accelerometer, a mass is suspended by two springs attached to opposite sides of the mass. The mass is maintained in a neutral position so long as the system is at rest or is in motion at a constant velocity. When the system undergoes a change in velocity in the direction of the springs' axis or perpendicular to the springs' axis, and therefore is accelerated in a particular direction, the spring mounted mass will at first resist the movement along that axis because of its inertia. This resistance to the movement, or delay in the movement, will force the springs to be temporarily either stretched or compressed. The tensile or compressive force acting on each spring is related to the product of the weight of the mass and the acceleration of the mass. The acceleration is then correspondingly determined by the change in velocity experienced by the mass.
Integrated circuit microaccelerometers having a proof mass suspended by pairs of microbridges are also known. An illustrative example of this type of accelerometer is disclosed in U.S. Patent application Ser. No. 07/304,057 to Chang et a "Resonant Bridge Two-Axis Accelerometer", now abandoned. In a microaccelerometer of this type, a proof mass is suspended by at least two pairs of microbridges. Each pair of microbridges is attached to opposite ends of the proof mass along a common axis. The acceleration of the mass is determined by the change in force acting upon each microbridge. This type of resonant microaccelerometer is attractive for precision measurements, because the frequency of a micromechanical resonant structure can be made highly sensitive to physical or chemical signals
A difficulty exists with regard to the manufacturing of these and other types of microaccelerometers The microbridges are typically formed from extremely thin layers of material, generally silicon, which are suspended over the silicon substrate. These thin layers, or beams, are difficult to manufacture. Similarly, an alternative type of sensor, a pressure sensor, is characterized by a solid thin diaphragm, wherein the same difficulties are experienced in trying to obtain the thin diaphragm. Many techniques have been used to micromachine these components, however there are shortcomings associated with these previous methods.
One common method has been to use an etchant, whereby the rate of etching is dependent upon the doping concentration of the silicon, to etch the silicon substrate and form the thin layers required for the membrane or microbridges. With this method a P+buried layer which is disposed under an N-type epitaxial layer acts as an etch stop when the silicon wafer is etched from the backside However, this method is problematic since a relatively thick layer of the N-type silicon epitaxy is required over the P+buried layer so as to ensure a sufficient amount of the device quality epitaxy after etching. This relatively thick layer of epitaxy directly affects the thickness and thus the sensitivity of the silicon microstructure. It would be desirable to provide a method for micromachining the silicon which would eliminate the P+etch stop, thereby eliminating the corresponding limit on the thickness of the silicon microstructure imposed by the quality of the overlaying epitaxial layer
Another method used to micromachine silicon microaccelerometers is to use a biased electrochemical etch wherein biased junction acts as the etch stop. A biased junction is formed at the interface between the P-type silicon substrate and overlaying N-type epitaxial layer. The biased junction serves as an etch stop when the silicon substrate is etched from the backside since the N-type epitaxial layer is passivated when exposed to the silicon etch. However, this method requires that electrical contact be made to each silicon wafer during etching of the silicon. Typically, there are many of these silicon microsensors formed on the silicon substrate, requiring constant electrical contact to each of the individual microsensors. Needless to say, maintaining these individual contacts is difficult in the harsh silicon etch environment. Therefore, it would be desirable to provide a method for micromachining the silicon wafer which does not require electrical contacts to be made.
It would also be desirable to provide a method for machining the silicon which provides more uniform dimensional control when forming the thin silicon layers, rather than silicon etch techniques which are relatively difficult to control. This is particularly desirable for automotive applications, where extremely large numbers of the sensors would be produced with the requirement that they all be dimensionally consistent.
Therefore, what is needed is a method for forming thin silicon membranes and beams which are suitable for use in a microaccelerometer or a pressure sensor which alleviates the above-mentioned detriments. In particular a method which utilizes an etch stop technique which does not unduly affect the dimensional characteristics of the microaccelerometer or pressure sensor.